Polymer dispersions for use in the adhesives and coatings sectors have been available primarily as solvent-containing formulations. For ecological reasons, however, aqueous polymer dispersions that can be processed to give corresponding aqueous coating or adhesive formulations have been introduced. Such aqueous dispersions include, for example, those based on polyvinylacetate (cf. Merkblatt TKH-3 “Dispersions-Holzleime” (wood-glue dispersions) issued 2004 by Industrieverband Klebstoffe eV, Düsseldorf (www.klebstoffe.com)).
Such known aqueous dispersions have the disadvantage that the layers have to be dried after application (open assembly time). In the case of adhesives, the expression “open assembly time” is understood, according to DIN 16920, to be the interval of time after applying the adhesive within which wet bonding is possible. This is the time after applying the adhesive up to the use of bonding pressure. The duration of bonding pressure until achieving sufficient initial, strength depends on a number of factors such as e.g. thickness applied, absorption capacity of the substrate, temperature while joining the substrates being bonded and the humidity of the air. In general the setting speed for aqueous adhesive formulations is slow and the film-formation temperature is restricted to temperatures >0° C. to >15° C., depending on the type of polymer Important characteristics of such formulations are therefore the open assembly time, the setting speed and the pot life of the dispersions, as well as the water resistance and thermal stability of the resulting dry coatings or adhesive films. The “pot life” is understood to be the time over which the formulation can be processed after incorporation of the second dispersion.
According to the prior art (Ullmann, Encyklopadie der techischen Chemie, vol. 14, 4th edition, p. 250), setting speed may be accelerated and the film-formation temperature may be lowered by adding solvents and/or plasticisers. However, the thermal stability of the coating or adhesive joint can be reduced by these additions. A higher thermal stability can be achieved by adding a second dispersion based on resorcinol or melamine resins or inorganic salts, such as, e.g. chromium nitrate. The “pot life” of these two-pack dispersion formulations, however, is restricted to a few hours.
Coatings and bonded joints with high water resistance and thermal stability can be obtained by the so-called “EPI system” (Emulsion Polymer Isocyanate). This is achieved by adding about 15% isocyanate, mostly MDI (diphenylmethane 4,4′-diisocyanate), to the polymer dispersion. However, due to the very short pot life, only mechanical application of two-pack formulations (2K-formulation) is possible.
In addition, various metal salt crosslinking agents used in such 2K-formulations are classified as corrosive or fire-promoting. In the case of isocyanate-based crosslinking agents, no matter what type of isocyanate, the irritant effect and the sensitizing potential towards skin and the respiratory tract have to be taken into account (see: Merkblatt TKH-3 “Dispersions-Holzleime” (wood-glue dispersions), issued 2004 by Industrieverband Klebstoffe eV, Dülsseldorf (www.klebstoffe.com).
The use of silica products in various other, unrelated applications is known, for example, the use of solid SiO2 products to control Theological properties, as fillers or as adsorbents. Silicon dioxide dispersions (for example silica sols), on the other hand, are used predominantly as binders for many different inorganic materials, as polishing agents for semiconductors or as flocculation partners in colloidal chemical reactions. In addition, for example, the use of polychloroprene lattices in the presence of silica sols as impregnation layers when producing flameproof elements is known. Pyrogenic silicas in combination with polychloroprene lattices for producing flame resistant foam finishes or bitumen finishes, and their use in combination with chloroprene/acrylic acid copolymers is also known.